Submarine Hydrothermal Systems Research Articles

Formation of hydrothermal ferromanganese oxides in the Daini-Nishi-Yamato Seamount, Sea of Japan: Do they really contain critical-metal particles?

In submarine environments, ferromanganese (Fe–Mn) oxides are formed via various processes. Although hydrothermal Fe–Mn oxides are generally valueless as mineral resources because of their low critical metal content, they reflect temporal changes in hydrothermal activity. Metal particles, including native metals, have been observed in hydrothermal Fe–Mn oxides and associated igneous rocks that are extensively distributed in the Sea of Japan. This finding can have significant implications for metal transport in submarine hydrothermal systems. However, the existence of these metal particles requires verification because their thermodynamic coexistence with Mn oxides is challenging and has only been reported by one research team. Here, we show the growth structure and geochemistry of hydrothermal Fe–Mn oxides and volcanic rocks from the Daini-Nishi-Yamato Seamount in the Sea of Japan. The chemical and Nd–Sr isotopic compositions of the volcanic rock indicated that the Daini-Nishi-Yamato Seamount formed through magmatism after back-arc spreading in the Yamato Basin at 10–17 Ma. The hydrothermal Fe–Mn oxides consisted of Ti-rich layers that formed earlier and Ti-poor layers that formed later. The Ti-rich layers precipitated from Ti-rich hydrothermal fluids because of water–rock interactions at higher temperatures during the early stages of hydrothermal activity. During the late stages of hydrothermal activity, Ti-poor layers formed, likely because of a decrease in the temperature of the water–rock interactions and/or the depletion of Ti in the host rocks. However, unlike in previous studies, metal particles such as Ag and rare earth elements were not detected in our samples, which were impregnated with resin to prevent contamination during the polishing process. These results strongly suggest that the metal particles reported in the previous studies were contaminants.

Effects of deep magmatic degassing and shallow seawater circulation on trace element and sulfur cycling in submarine hydrothermal systems: Insights from the Shijuligou analog, North China

Limited access to modern subseafloor sulfides hampers our understanding of the link between magmatic volatile influx and the cycling of trace elements and sulfur, as well as the effect of the subsequent shallow seawater circulation on these processes. Hence, we studied a well-preserved fossil analog of submarine hydrothermal systems – the Shijuligou volcanogenic massive sulfide (VMS) deposit from the North Qilian Mountains in North China to examine variations in elements and isotopes of subseafloor sulfides vertically.The vertical distribution of trace elements in subseafloor sulfides is strongly controlled by temperature gradients and redox states during the interaction between hot fluids and seawater beneath the paleo-seafloor. While the enrichment of elements like As, Sb, and Au (median: 1335, 43.7, and 0.30 ppm, respectively, n = 21) and negative δ34S values (mean: −3.07 ‰, n = 7) of euhedral pyrites in the jasper, along with the precipitation of high sulfidation minerals (e.g., enargite), suggest the input of magmatic volatiles into hydrothermal systems. During the shallow seawater-hydrothermal circulation, pyrites in the veined and stockwork zones exhibit distinctly elevated δ34S values (up to 15.74 ‰), accompanied by increased concentrations of wall-rock-derived elements (e.g., Cu, Ni, Si, and Ti) and low-temperature-responsive elements (e.g., Pb, Zn, and Cd). Sulfur isotopes of sulfides vary significantly from the surface to the deep ore zones, ranging from −3.36 to 19.84 ‰ (mean: 9.25 ‰, n = 37). The negative δ34S values of pyrites at the paleo-seafloor are due to the addition of H2S derived from the disproportionation of magmatic SO2. The increased δ34S values of stockwork and disseminated sulfides at depth are attributed to the progressive reduction of seawater sulfates by ferrous iron released from the alteration of fresh basalts. The trace elemental and isotopic characteristics of sulfides suggest the Shujuligou VMS deposit resembles the fossil analog of immature, subduction-related submarine hydrothermal systems.

Numerical Simulation of Chemical Reactions’ Influence on Convective Heat Transfer in Hydrothermal Circulation Reaction Zones

Chemical reactions, mineral diffusion, and deposition are pivotal in understanding the mechanisms of mineral deposition and the formation of seafloor sulfides in the hydrothermal circulation process. To understand the formation process of anhydrite in submarine hydrothermal systems, a computational model that combined component transport and chemical reactions was established and simulated using the mass transport model. The deposition rate of calcium sulfate was defined, and the effects of factors such as porosity, ion concentration, and inflow velocity on the temperature field in the reaction zone were thoroughly investigated. The distribution of temperature, porosity, and velocity during the reaction process was obtained, allowing for the identification of the chemical reaction patterns of certain ions in the early stages of hydrothermal activity. The simulation revealed the occurrence of biochemical reactions between two types of ions, leading to their deposition on the solid framework of a porous medium. With the increase in inflow velocity and solute concentration, the average porosities of the porous medium decreased by 0.495% and 0.468%, respectively, which consequently altered the structure of the rock. Such findings contribute to the inference of formation and extinction mechanisms of seafloor crusts and hydrothermal chimneys.

Magnesium silicate chimneys at the Strytan hydrothermal field, Iceland, as analogues for prebiotic chemistry at alkaline submarine hydrothermal vents on the early Earth

The Strytan Hydrothermal Field (SHF) in basaltic terrain in Iceland is one of the extant alkaline submarine hydrothermal vent systems favoured as analogues for where life on Earth may have begun. To test this hypothesis we analyse the composition, structure, and mineralogy of samples from hydrothermal chimneys generated at the SHF. We find that the chimney precipitates are composed of Mg-silicates including clays of the saponite-stevensite group (high Mg and Si, low Fe and Al), Ca-carbonates and Ca-sulfates. The chimneys comprise permeable structures with pores sizes down to 1 µm or less. Their complex interiors as observed with SEM (Scanning Electron Microscopy) and X-ray CT (computed tomography scanning), exhibit high internal surface areas. EDX (energy-dispersive X-ray spectroscopy) analysis reveals an increase in the Mg/Si ratio toward the chimney exteriors. Chemical garden analogue experiments produce similar Mg–silicate chimneys with porous internal structures, indicating that injection-precipitation experiments can be high-fidelity analogues for natural hydrothermal chimneys at the SHF. We conclude that SHF chimneys could have facilitated prebiotic reactions comparable to those proposed for clays and silica gels at putative Hadean to Eoarchean alkaline vents. Analysis of the fluid dynamics shows that these chimneys are intermediate in growth rate compared to faster black smokers though slower than those at Lost City. The SHF is proposed as a prebiotic alkaline vent analogue for basaltic terrains on the early Earth.

Fossil hydrothermal oceanic systems through in-situ B isotopes in ophicarbonates (N. Apennines, Italy)

In submarine-hydrothermal systems, fluid-rock interactions play a pivotal role in fostering life on rocky planets. Carbonated-ultramafic rocks (i.e., ophicarbonates) represent an important witness of such environments and their study may provide new insights into hydrothermal processes. Here, we report in-situ trace element concentrations and the first in-situ boron isotope analyses (δ11B) of serpentines from the Ligurian N. Apennine (Italy) ophicarbonates, which represent non-subducted remnants of Jurassic ultramafic-hosted hydrothermal system. Based on micro-Raman analyses, lizardite is the dominant stable serpentine polymorph in the studied samples, thus constraining the temperature (T) of serpentinization below ca. 320 °C. By means of UPb LA-ICP-MS calcite geochronology, the age of the carbonation event is constrained at 137 ± 3 Ma (2σ, n = 52). The increasing inventory of several fluid-mobile elements (e.g., B, Li, As, Sb, Sr, U) associated with the REE budget variability in serpentines from ophicarbonates compared to those in pure serpentinites suggest that physico-chemical changes during the evolution of the hydrothermal system exert a relevant influence on the geochemistry of serpentines. Pure serpentinites, which are considered the protoliths of the ophicarbonates, show positive δ11B signatures from +15.8 to +35.0‰ (n = 24), thus falling in the range of present-day oceanic and forearc serpentinites and ophiolite-derived serpentinites. The δ11B imprint of serpentines in ophicarbonates clusters at +10.2 ± 2.2‰ (2SD, n = 57) and − 5.7 ± 1.5‰ (2SD, n = 36). Remarkably, this is the first report of oceanic serpentines showing negative B isotope compositions. The positive δ11B data likely reflect inheritance from the precursor serpentinite, while the negative δ11B values are here related to the hydrothermal process(es). We attempt to model how the variation in pH and different δ11B composition of the hydrothermal vent fluids (+25 and + 8‰) at different T (100 and 150 °C) can affect the δ11B imprint of serpentines. Our results indicate that either pH condition or δ11B of the hydrothermal vent fluids can shift the δ11B of serpentines from positive to negative values. Our new trace element and δ11B data are integrated and discussed with those from the literature to achieve new constraints in the variations and controls on the physical-chemical conditions of fluid-rock interaction during the progressive evolution of long-lived fossil ultramafic-hosted hydrothermal system. Finally, we discuss how subduction processes can potentially modify the B geochemistry of ophicarbonates and the information that we could achieve by studying B isotopes in such high-P lithologies to unravel the B- and C-cycles into the Earth's mantle since the onset of modern plate tectonics.

Sulfur isotopes as a new indicator for the maturation of sulfide chimneys in submarine hydrothermal systems

Understanding the relationship between sulfide chimney maturity and fluid evolution in submarine hydrothermal systems is essential for deciphering mineralogical changes, sulfur dynamics, and fluid flux over time. We investigate this link through sulfur isotopic analysis of sulfides from three maturity stages of sulfide chimneys at the Niaochao hydrothermal field (East Pacific Rise, 1–2°S), integrated with δ34S values from diverse global hydrothermal fields.Based on mineral associations and sulfur isotopes, the sulfide chimneys are classified into Zn-Fe-rich, Fe-Cu-rich, and Fe-Cu-Zn-rich chimneys, representing the increasing maturity. A systematic increase in δ34S values (from 1.54‰ to 7.22‰) from chimney cores to rims indicates the progressive reduction of seawater sulfate during fluid-seawater mixing in open systems. Fluctuations in δ34S values within chimney zones are attributed to the interaction between hydrothermal fluids and pre-existing minerals. As chimneys grow, rapid sulfur isotopic exchanges occur between seawater sulfate and fluid H2S in localized semi-closed systems, leading to a significant drop in δ34S values of pyrites, decreasing by up to 4.82‰.A new chimney evolutionary model has been developed utilizing mineralogical and sulfur isotopic data of sulfide chimneys from global hydrothermal vents. This model encompasses birth, growth, and waning stages, accompanied by an elevation of mean δ34S values from 2.1‰ (n = 66) to 3.2‰ (n = 435) and 3.8‰ (n = 61) due to ongoing seawater sulfate reduction. Conversely, the infusion of magmatic sulfur during chimney growth contributes to a decrease in δ34S values of sulfides. This study underscores the significance of sulfur isotopes as indicators of sulfide chimney maturity, which enhances our understanding of the evolution and formation processes of hydrothermal systems.

Nature and origin of primary ore-forming fluids in the highly metamorphosed Archean Hongtoushan VMS deposit, North China: Insights from in situ S isotopes and trace elements of sulfides

Many ancient volcanogenic massive sulfides (VMS) deposits underwent various degrees of deformation and metamorphism, which modified the primary orebody geometries and sulfide textures, resulting in the great challenge of revealing the source and physicochemical properties of primary ore-forming fluids and original mineralization processes. Here, we observed the textural evolution of sulfides in the highly metamorphosed Archean VMS Hongtoushan Cu-Zn deposit, China, to constrain the histories and P-T conditions of metamorphic remobilization and to evaluate the influence of metamorphism on geochemical characteristics of sulfide. Further, the in situ sulfur isotopic and trace elemental analyses were utilized to reveal the source and physicochemical conditions of initial fluids. During metamorphism, pyrite developed elongated texture, bulging recrystallization, coarse porphyroblasts, and foam texture. These textures all occurred at a temperature of 550 – 700 °C, at which sulfide remobilization and deformation occurred in an anhydrous environment. The geochemical exchange among sulfides did not happen during metamorphism. The sulfur isotope and trace elements of sulfides thus can be used to track the source and physicochemical properties of primary ore-forming fluids. In situ sulfur isotopic analyses of sulfides yielded values of δ34S approaching zero and Δ33S situating mass-dependent isotope fractionation (MDF) area, indicating that the sulfur of the original submarine hydrothermal system was mainly derived from the magmatic fluids. The content of Se in pyrite revealed the primary fluids were acidic and reductive and were used to constrain the temperature of ore-forming fluids, which was higher than 250 °C. The ratio of Co/Ni of pyrite was higher than 10, indicating primary fluids were of magmatic-hydrothermal origin. The high content of Se and As in pyrite and Cd and Hg in sphalerite documented that these elements were partially derived from the magmatic volatiles or magma degassing process. Overall, our study demonstrates that coupling textures and in situ geochemical features of sulfides could be a helpful tool for tracking the primary ore-forming processes of the metamorphosed massive sulfide deposits.

Extreme accumulation of ammonia on electroreduced mackinawite: An abiotic ammonia storage mechanism in early ocean hydrothermal systems.

An increasing amount of evidence suggests that early ocean hydrothermal systems were sustained sources of ammonia, an essential nitrogen species for prebiotic synthesis of life's building blocks. However, it remains a riddle how the abiotically generated ammonia was retained at the vent-ocean interface for the subsequent chemical evolution. Here, we demonstrate that, under simulated geoelectrochemical conditions in early ocean hydrothermal systems ([Formula: see text][Formula: see text] V versus the standard hydrogen electrode), mackinawite gradually reduces to zero-valent iron ([Formula: see text]), generating interlayer [Formula: see text] sites. This reductive conversion leads to an up to 55-fold increase in the solid/liquid partition coefficient for ammonia, enabling over 90% adsorption of 1 mM ammonia in 1 M NaCl at neutral pH. A coordinative binding of ammonia on the interlayer [Formula: see text] sites was computed to be the major mechanism of selective ammonia adsorption. Mackinawite is a ubiquitous sulfide precipitate in submarine hydrothermal systems. Given its reported catalytic function in amination, the extreme accumulation of ammonia on electroreduced mackinawite should have been a crucial initial step for prebiotic nitrogen assimilation, paving the way to the origin of life.

Highly diverse diazotrophs drive high N2 fixation rates in a shallow submarine hydrothermal system

Dinitrogen (N2) fixation provides nitrogen sources supporting active chemoautotrophic processes in submarine hydrothermal systems. Here, the diazotrophic phylogenetic diversity and N2 fixation profile were investigated in depth along a sharp biogeochemical gradient from the hydrothermal vents to ambient seawater in a shallow submarine hydrothermal system (SHS) off Kueishantao Islet, Taiwan. Diazotrophic community compositions in the SHS covered all four known nifH clusters (I, II, III, and IV) and contained both characteristics of diazotrophic communities in deep-sea hydrothermal systems and terrestrial geothermal systems. The seawater around the vents was primarily dominated by nifH phylotypes affiliated with chemolithotrophic sulfur-oxidizing and iron-reducing bacteria. In contrast, in the seawater directly above the hydrothermal vents, the nifH community markedly contained diverse heterotrophic bacteria. The analysis, which focused on amino acid composition, protein spatial structure, and the possible co-occurrence of alternative nitrogenases in the presence of molybdenum-nitrogenase, further revealed that diazotrophic nitrogenase found around the hydrothermal vents exhibits potential molecular adaptations to high temperature, low pH, and sulfur-rich conditions. The relatively low N2 fixation rates (NFRs) were observed around the vents, where high concentrations of inorganic nitrogen compounds were present. However, notably, high NFRs were detected in seawater directly above the hydrothermal vents, which were 2–5 times higher than those at the reference site. The abundant iron, along with the depletion of nitrogen nutrients potentially resulting from nitrogen assimilation and loss processes, may have led to a high iron:nitrogen ratio, inducing the observed high NFRs. The co-occurrence of diverse nifH-containing heterotrophic bacteria and high NFRs suggests that these heterotrophs may play a major role in N2 fixation in this environment. This study suggests that the SHSs may be potential hotspots for marine N2 fixation, contributing significantly to the global ocean's N2 fixation budget and warranting further investigation.

Experimental evidence for the hydrothermal formation of native sulfur by synproportionation

Elemental sulfur (S0) is known to form in submarine acid-sulfate vents by disproportionation of magmatic SO2. S0 formed upon disproportionation shows δ34SS values considerably lower than the influxing magmatic SO2, which results in δ34SS values typically <0‰. The peculiar occurrence of isotopically heavy sulfur in the Kemp Caldera hydrothermal system (δ34SS > 5‰) and Niua North (δ34SS = 3.1‰) led to the suggestion that disproportionation is not the only sulfur forming process in submarine hydrothermal systems. We conducted hydrothermal experiments to investigate if synproportionation of SO2 and H2S can explain the occurrence and isotopic composition of S0 observed in some vent fields. Provided that SO2 and H2S are both abundant, this formation mechanism is thermodynamically conceivable, but it has not yet been demonstrated experimentally that this process actually takes place in submarine hydrothermal systems. We conducted the experiments in collapsible Ti-cells under pT-conditions (20–30 MPa, 220°C) that are relevant to S0 formation in submarine hydrothermal systems. We used starting concentrations of 10 mM sulfite and 20 mM sulfide of known isotopic composition. Under acidic conditions (pH25 °C = 1.2), S0 was the most abundant reaction product, but small amounts of sulfate were also produced. A Rayleigh fractionation model was applied to determine the isotopic composition of SO42–, SO2, H2S and S0 expected to form by SO2 disproportionation, H2S oxidation, and SO2–H2S synproportionation. The sulfur isotopic signatures of the sulfur produced in the experiments can only be explained by synproportionation of sulfite and sulfide. These results provide strong evidence that synproportionation is likely responsible for exceptionally high δ34SS values observed in S0 from some arc/back-arc hydrothermal environments, like the Kemp Caldera in the South Sandwich arc. Coeval degassing of H2S and SO2 is likely required to have this particular reaction dominate in the H–S–O reaction network and produce noticeable accumulations of isotopically heavy native sulfur at the seafloor.

Occurrence and precipitation mechanism of silver in pyrite from chimney fragments in the Edmond hydrothermal field, Central Indian Ridge

The enrichment of precious elements including Au and Ag in submarine hydrothermal sulfide deposits attracts more and more attention. Previous studies indicate that Au and Ag combine distinctly different ligands under the physico-chemical conditions of submarine hydrothermal systems, therefore, the factors controlling their precipitation may be different. In general, silver mineralization has been much less studied than gold mineralization in submarine hydrothermal sulfide deposits. The Ag mineralization process and precipitation mechanism of submarine hydrothermal sulfide deposits formed at Mid-ocean Ridges are still poorly constrained. In this study, we studied the occurrence and precipitation mechanism of Ag in sulfide deposits of Edmond hydrothermal field, located on the intermediate-spreading Central Indian Ridge (CIR).Three ore-forming stages were identified in chimney fragments collected in the Edmond hydrothermal field. The corresponding minerals are (stage I) anhydrite + barite + colloidal/porous pyrite (Py1) + fine-grained sphalerite; (stage II) marcasite; (stage III) euhedral pyrite (Py2) + coarse-grained sphalerite + chalcopyrite + isocubanite. Py1, characterized by colloidal, porous, and micro-sized anhedral morphology, was usually overgrown by marcasite, which is, in turn, surrounded by euhedral-subhedral Py2, usually coexisting with coarse-grained sphalerite, chalcopyrite, and isocubanite. Compared to marcasite and Py2, only Py1 contains abundant native silver nanoparticles. LA-ICP-MS analyses suggest that Py1 contains higher Pb, Cu, Ag, Mn, Tl, Mo, Au, and lower Co as compared to Py2. In-situ LA-ICP-MS and nanoSIMS analyses indicate that Py1 with higher Ag contents (101–586 ppm) has larger variation in sulfur isotopic compositions (0.8 to 6.6‰) than those of Py2 with lower Ag contents (0.22–15.5 ppm; δ34S values: 2.1 to 4.8‰ for Fe-rich samples and 0 to 2.8‰ for Zn-rich samples).Texture, mineral assemblage, pyrite trace element and sulfur isotopic compositions indicate a progressive decrease on the degree of fluid-seawater mixing as the temperature gradually increases during the growth of the chimneys in Edmond hydrothermal field. Phase diagram analysis indicates that the increase in pH and in particular cooling, due to the mixing of the hot hydrothermal fluid with cold seawater, can significantly decrease the solubility of Ag and be the effective mechanisms of silver precipitation. Our results suggest that mixing of hydrothermal fluid with seawater is the main Ag precipitation mechanism for submarine hydrothermal sulfide deposits formed at Mid-ocean Ridges.

Effects of Hydrothermal Alteration and Mineralization on the Paleomagnetic Properties of Rocks from IODP Expedition 376 at Brothers Volcano

AbstractThe 3-D subseafloor architecture of submarine hydrothermal systems is largely unknown, particularly at arc volcanoes. The alteration of volcanic rocks in these systems produces dramatic changes in their magnetic properties. Here, we present the first comprehensive study of paleomagnetic measurements from oriented samples of hydrothermally altered dacites from Brothers volcano (Kermadec arc), drilled during International Ocean Discovery Program (IODP) Expedition 376. These data have enabled insight into the progressive evolution of magnetic minerals in subseafloor volcanic rocks affected by variable types and degrees of hydrothermal alteration in response to varying fluid temperatures, chemistry, and associated mineralization; from initial chloritization typical of relatively low-temperature interaction with seawater to extremely altered rocks affected by higher-temperature, very acidic magmatic fluids.Hydrothermally altered samples show a significant reduction in natural remanent magnetization intensity (10–4 to 10–2 A/m) compared with unaltered samples (1–10 A/m), suggesting that primary titanomagnetite grains are destroyed during the hydrothermal alteration process. Except for a small region in proximity to the mineralized stockwork zone, no chemical remanent magnetization is observed in association with hydrothermal alteration, consistent with the widespread formation of diamagnetic and/or paramagnetic minerals such as pyrite, rutile, and leucoxene, which do not carry any natural remanent magnetization.Demagnetization experiments show that most of the oriented samples possess a stable characteristic remanent magnetization induced by the residual primary magnetic minerals formed at the time the rocks cooled on the sea floor. Partially chloritized dacites, however, are characterized by large magnetic susceptibilities, low Koenigsberger ratios, and very low magnetic coercivities, consistent with initial dissolution of smaller, singledomain magnetic grains, indicating that intensely hydrothermally altered rocks are better paleomagnetic indicators than initially chloritized samples at the periphery of the hydrothermal systems.The significant magnetic contrast between fresh and hydrothermally altered rocks, in addition to a thick layer (>300 m) of demagnetized rocks observed at Brothers volcano, confirms the empirical results that magnetic anomalies are important geophysical tools to determine the geometry of hydrothermal systems at submarine arc volcanoes.

Elemental sulfur, hydrogen sulfide, and carbon monoxide-driven dimerization of glycine at sub-millimolar concentrations: Implications for the origin of life

The life’s origin in submarine hydrothermal systems is a long-standing scenario supported by diverse scientific disciplines, while verification of its chemical plausibility remains a major bottleneck. Most scenarios of hydrothermal origin postulate peptides as key players in prebiotic chemistry; nevertheless, it remains unknown how peptides were formed from geochemically available amino acids that must be low in concentration. Here we show that a simple mixing with elemental sulfur (S0), hydrogen sulfide (HS–), and carbon monoxide (CO) enables an effective dimerization of glycine (Gly) at micro to several millimolar concentrations. Incubation of 1 mM Gly with the three inorganic compounds in a warm alkaline solution (pH 9.3 and 35 or 50 °C) led up to 18% conversion of Gly to glycylglycine (GlyGly). Dimerization of 0.01 mM Gly was also discerned in the yield far exceeding the thermodynamic equilibrium (0.4% yield). In this reaction, CO is oxidized to CO2 concomitant with the reduction of S0 to polysulfides, serving as the driving force for the Gly-to-GlyGly conversion. Although the formation of CO2 as a byproduct suppressed the formation of GlyGly, its adverse effect was mitigated by carbonate precipitation upon adding Mg2+ and Ca2+. To our knowledge, 0.01 mM is the lowest concentration of amino acid oligomerized experimentally in the prebiotic context. Given the ubiquity of S0, HS–, and CO in primordial ocean hydrothermal systems, and the abundance of Mg2+ and Ca2+ in seawater, our demonstrated favorable condition for amino acid dimerization is likely to have occurred on the Hadean seafloor. This possibility supports an abiotic appearance of peptides and thereby facilitates the hydrothermal origin of life.

The influence of submarine hydrothermal systems on seawater sulfate

Seawater sulfate is a major carrier of oxidizing capacity and influences the redox budget of the Earth’s surface on geologic timescales. Records of oxygen and sulfur isotopes in seawater sulfate are used to track changes in sulfate cycling through geologic time. Interpretations of these records are typically based on models that describe the seawater sulfate reservoir as a mass balance between microbial, riverine and sedimentary sulfate fluxes. Here, we investigate the influence of hydrothermal sulfate cycling, which remains an unconstrained but potentially significant additional flux in this mass balance. We find that anhydrite (CaSO4) from eight submarine hydrothermal vent fields is consistently offset from seawater sulfate in δ18O but not Δ’17O or δ34S. Experiments at hydrothermal pressure–temperature conditions indicate that this δ18O offset is driven by oxygen isotope exchange between sulfate and hydrothermal H2O at high temperature. An updated isotope mass balance model shows that a flux of hydrothermal sulfate into seawater, derived from retrograde dissolution of hydrothermal anhydrite, could cause high-temperature oxygen isotope exchange to buffer seawater sulfate δ18O and Δ’17O by as much as 25%. Hydrothermal sulfate preserved in the Troodos Ophiolite (Cretaceous) also records a δ18O and Δ’17O signal of high-temperature oxygen isotope exchange, supporting the conclusion that geologic records of seawater sulfate oxygen isotopes may include a hydrothermal component.

Cyclic tidalites and seismites at a submarine hydrothermal system for a 2450 Ma banded iron formation, Hamersley Basin, Western Australia

The Hamersley Group in the Hamersley Basin, Western Australia, contains major deposits of banded iron formation of Neoarchean–Paleoproterozoic age (2629–2449 Ma). The 2450 Ma Weeli Wolli Formation near the top of the group includes banded iron formation with microbands (≤0.05–0.5 mm to rarely 1 mm thick) of chert and iron oxide lamina couplets that display distinctive cycles. Some adjacent, thin microbands have merged with compaction and the diagenetic removal of chert laminae, and maximum counts of cycle period by different studies is preferred, suggesting a period of ∼28 ± 2 microbands. Associated cyclic iron formation shows soft-sediment plastic deformation and brittle fracturing. Rare earth element geochemistry indicates that much of the iron in late Archean–early Paleoproterozoic banded iron formations was derived from submarine hydrothermal systems. Monitoring of hydrothermal vents on mid-ocean ridges and seamounts has identified tidal modulation of the discharge, temperature and dispersal of deep-sea hydrothermal plumes principally by semidiurnal variations in tidal loading and earth tides. Microearthquake swarms are associated with deep-sea hydrothermal systems at the crests of mid-ocean ridges, and tidal triggering of microearthquake activity is indicated for a hydrothermal system on the East Pacific Rise. The microband cycles in the Weeli Wolli Formation are interpreted as tidalites, comprising ∼28 ± 2 semidiurnal microbands per lunar (synodic) fortnightly cycle, related to the tidally modulated activity of a submarine hydrothermal system. The associated plastically deformed and brittle-fractured sediments are viewed as seismites resulting from microearthquakes also related to the hydrothermal system. The Weeli Wolli microband cycles and seismites may represent a rarely identified instance of iron-formation deposition near a submarine hydrothermal system. KEY POINTS Submarine hydrothermal venting was a major source of iron in late Archean–early Paleoproterozoic banded iron formations. Semidiurnal variations in tidal loading and earth tides commonly modulate present-day deep-sea hydrothermal activity, and microearthquake swarms are associated with deep-sea hydrothermal systems. Early Paleoproterozoic banded iron formation from the Hamersley Basin displays cycles with an estimated period of ∼28 ± 2 microbands and associated plastic deformation and brittle fracturing. The microband cycles are interpreted as tidalites comprising ∼28 ± 2 semidiurnal increments grouped in fortnightly cycles at a tidally modulated submarine hydrothermal system, and the deformed sediments as seismites caused by related microearthquake activity.

Microbial ecology of a shallow alkaline hydrothermal vent: Strýtan Hydrothermal Field, Eyjafördur, northern Iceland.

Strýtan Hydrothermal Field (SHF) is a submarine system located in Eyjafördur in northern Iceland composed of two main vents: Big Strýtan and Arnarnesstrýtan. The vents are shallow, ranging from 16 to 70 m water depth, and vent high pH (up to 10.2), moderate temperature (T max ∼70°C), anoxic, fresh fluids elevated in dissolved silica, with slightly elevated concentrations of hydrogen and methane. In contrast to other alkaline hydrothermal vents, SHF is unique because it is hosted in basalt and therefore the high pH is not created by serpentinization. While previous studies have assessed the geology and geochemistry of this site, the microbial diversity of SHF has not been explored in detail. Here we present a microbial diversity survey of the actively venting fluids and chimneys from Big Strýtan and Arnarnesstrýtan, using 16S rRNA gene amplicon sequencing. Community members from the vent fluids are mostly aerobic heterotrophic bacteria; however, within the chimneys oxic, low oxygen, and anoxic habitats could be distinguished, where taxa putatively capable of acetogenesis, sulfur-cycling, and hydrogen metabolism were observed. Very few archaea were observed in the samples. The inhabitants of SHF are more similar to terrestrial hot spring samples than other marine sites. It has been hypothesized that life on Earth (and elsewhere in the solar system) could have originated in an alkaline hydrothermal system, however all other studied alkaline submarine hydrothermal systems to date are fueled by serpentinization. SHF adds to our understandings of hydrothermal vents in relationship to microbial diversity, evolution, and possibly the origin of life.

Using thermodynamics to obtain geochemical information from genomes.

Thermodynamic characterization of the relative stabilities of chemical compounds is a pillar of conceptual models in various fields of geosciences. Analogous models applied to genomes can yield new information about the relationship between genomes and their geochemical environments. In this perspective article, we present a chemical and thermodynamic analysis of prokaryotic lineages that have been the target of previous phylogenomic studies of evolutionary adaptation to varying redox conditions. The thermodynamic model development begins by quantifying the effects of hydrogen activity (aH2 ) and temperature on the relative stabilities of organic compounds with different carbon oxidation state. When applied to proteins instead of metabolites, the same techniques can be used to identify combinations of aH2 and temperature at which reference proteomes for Class I or Class II methanogens are relatively stable. The calculated aH2 values are compatible with reported measurements for habitats of methanogens ranging from highly reducing submarine hydrothermal systems to less reducing environments including methanogenic sediments. In contrast to the transition between the two classes of methanogenic archaea, that between basal and terrestrial groups of Thaumarchaeota (denoting the origin of ammonia-oxidizing archaea) occurs at a less-reducing redox boundary. These examples reveal the consequences of energy minimization driving evolution and show how geochemical calculations involving biomolecules can be used to quantify and better understand the coevolution of the geosphere and biosphere.

Trace-element distribution and ore-forming processes in Au–Ag-rich hydrothermal chimneys and mounds in the TA25 West vent field of the Tonga Arc

We report detailed mineralogy and geochemistry of hydrothermal mounds and chimneys in the TA25 West vent field (TA25 WVF), a newly discovered magmatic–hydrothermal system in the Tonga (Tofua) arc. Chimney samples are classified as sulfate- or sulfide-rich, based on major sulfide, sulfosalt, and sulfate minerals. The former type represents a simple mineral assemblage of predominance of anhydrite/gypsum + barite + pyrite, whereas the sulfide-rich chimneys show three different stages of mineralization with decreasing fluid temperature: sphalerite–pyrite dominated stage I, sphalerite–sulfosalts dominated stage II, and stage III is dominated by seawater alteration. Mound samples are characterized by sulfide assemblages and paragenesis similar to those of sulfide-rich samples, but abundant chalcopyrite indicates a relatively high-temperature mineralization. The chimney and mound samples are enriched in Au (average 9.2 ppm), Ag (297 ppm), As (1897 ppm), Sb (689 ppm), Hg (157 ppm), and Se (34.6 ppm). LA–ICP–MS and FE–TEM studies indicate that most of these elements occur in sulfides or sulfosalts in solid solution, although some occur as nanoparticles. This is mainly controlled by the combined effects of fluid conditions (temperature and redox state) and influx of ambient seawater. Petrography and trace-element compositions of sulfides and/or sulfosalts suggest that most concentrations of Au and Ag in the TA25 WVF result from the precipitation and/or adsorption of Au–Ag-bearing nanoparticles on rapidly crystallized sulfides, the substitution of Au and Ag in sulfide and/or sulfosalt minerals, and the saturation of Ag in hydrothermal fluids during late, relatively low-temperature mineralization (< 150 °C). The maximum measured temperature (242 °C) of venting fluids and calculated formation temperatures of sphalerite (229–267 ℃) are below the boiling temperature of seawater at the depths (966–1096 m) of the TA25 WVF, suggesting fluid boiling had little effect on Au–Ag-rich mineralization in the TA25 WVF. The presence of enargite–tetrahedrite–tennantite assemblages, high concentrations of magma-derived elements (e.g., Au, Ag, As, Sb, Hg, and Se), low δ34S values (2.1 to 4.3‰) of sulfide minerals relative to the host rocks, and the distribution of CO2-rich hydrothermal plumes (500 to 1000 ppm) suggest that the TA25 WVF is a submarine hydrothermal system influenced by a magmatic contribution in an arc setting. Our results indicate that the magmatic contribution is most likely to play an important role in supplying various metals, including Au and Ag, to the TA25 WVF. Subsequently, the rapid crystallization of sulfides induced by abundant fluid-seawater mixing significantly contributes to the precipitation of Au–Ag-rich mineralization.

From Fossil to Active Hydrothermal Outflow in the Back‐Arc of the Central Apennines (Zannone Island, Italy)

AbstractPost‐orogenic back‐arc magmatism is accompanied by hydrothermal ore deposits and mineralizations derived from mantle and crustal sources. We investigate Zannone Island (ZI), back‐arc Tyrrhenian basin, Italy, to define the source(s) of mineralizing hydrothermal fluids and their relationships with the regional petrological‐tectonic setting. On ZI, early Miocene thrusting was overprinted by late Miocene post‐orogenic extension and related hydrothermal alteration. Since active submarine hydrothermal outflow is reported close to the island, Zannone provides an ideal site to determine the P‐T‐X evolution of the long‐lived hydrothermal system. We combined field work with microstructural analyses on syn‐tectonic quartz veins and carbonate mineralizations, X‐ray diffraction analysis, microthermometry and element mapping of fluid inclusions (FIs), C, O, and clumped isotopes, and analyses of noble gases (He‐Ne‐Ar) and CO2 content in FIs. Our results document the evolution of a fluid system of magmatic origin with increasing mixing of meteoric fluids. Magmatic fluids were responsible for quartz veins precipitation at ∼125 to 150 MPa and ∼300°C–350°C. With the onset of extensional faulting, magmatic fluids progressively interacted with carbonate rocks and mixed with meteoric fluids, leading to (a) host rock alteration with associated carbonate and minor ore mineral precipitation, (b) progressive fluid neutralization, (c) cooling of the hydrothermal system (from ∼320°C to ∼86°C), and (d) embrittlement and fracturing of the host rocks. Both quartz and carbonate mineralizations show noble gases values lower than those from the adjacent active volcanic areas and submarine hydrothermal systems, indicating that the fossil‐to‐active hydrothermal history is associated with the emplacement of multiple magmatic intrusions.

Biomarkers in the Atacama Desert along the moisture gradient and the depth in the hyperarid zone: Phosphatase activity as trace of microbial activity

AbstractMicrobiological activities can be detected in various extreme environments on Earth, which suggest that extraterrestrial environments, such as on Mars, could host life. There have been proposed a number of biomarkers to detect extant life mostly based on specific molecules. Because terrestrial organisms have catalytic proteins (enzymes), enzymatic activity may also be a good indicator to evaluate biological activities in extreme environments. Phosphatases are essential for all terrestrial organisms because phosphate esters are ubiquitously used in genetic molecules (DNA/RNA) and membranes. In this study, we evaluated microbial activity in soils of the Atacama Desert, Chile, by analysing several biomarkers, including phosphatase activity. Phosphatases extracted with Tris buffer were assayed fluorometrically using 4-methylumbelliferyl phosphate as a substrate. The horizontal distribution of phosphatase activity and other parameters in soils from the Atacama Desert showed that phosphatase activity was positively correlated with amino acid concentration and colony-forming units and negatively correlated with precipitation amount. We found consistent that biochemical indicators including phosphatase significantly decreased in the extreme hyper-arid zone where rainfall of &lt;25 mm year−1. The results were compared with phosphatase activities detected in extreme environments, such as submarine hydrothermal systems and Antarctic soils, as well as soils from ordinary environments. Overall, our results suggested that phosphatase activity could be a good indicator for evaluating biological activities in extreme environments.